github/workflow: switch back neovim version to nightly

Signed-off-by: Koichi Shiraishi <zchee.io@gmail.com>

Author: zchee

.github/workflows/test.yml

@@ -64,7 +64,7 @@ jobs:
       if: steps.cache-nvim.outputs.cache-hit != 'true' || steps.cache-nvim-windows.outputs.cache-hit != 'true'
       with:
         neovim: true
-        version: v0.5.0
+        version: nightly
 
     - name: gofmt
       if: ${{ env.OS != 'windows' }}